This invention relates to face seal assemblies in general, and specifically to such an assembly intended for use in a vehicle water pump.
Vehicle water pumps have an impeller drive shaft that extends through a generally circular opening in a housing filled with cooling fluid. The annular space or gap created between the shaft and housing opening must be sealed against leakage. A primary seal face is fixed to either the housing opening or to the outer surface of the shaft, with a contact surface that is normal to the shaft axis. The face seal assembly has a mounting member, typically a metal sleeve, that is installed to the other of the housing opening or the outer surface of the shaft, and a secondary seal face that contacts the primary seal face and rotates relative to it. Therefore, the face seal assembly has two basic interfaces that must be kept fluid tight, the installation interface and the seal face contact interface.
There is no relative rotation at the installation interface, so it is relatively easy to maintain it fluid tight. However, the contact interface involves very rapid relative rotation, besides being exposed to a hot, corrosive fluid and a high degree of abrasive grit, and has historically proved difficult to seal. The need to retain coolant is such that a high degree of seal torque has been acceptable at the contact interface in return for maintaining the integrity of the seal. Seal integrity at the contact interface is generally maintained by an axially resilient spring that forces the primary and secondary seal faces strongly and continually together. The spring also provides axial wear take up at the contact interface. Wear take up, in turn, requires a degree of relative axial movement with time between the secondary seal face and the mounting member, so they can not be axially fixed relative to one another. Generally, an axially flexible sealing member has been used to provide the relative axial motion needed for wear take up. The flexible sealing member is usually elastomer, and often referred to as a bellows or a boot.
The addition of the flexible sealing member to the assembly creates other potential problems that must be dealt with. Since the secondary seal face is not rigidly, directly joined to the mounting member, some means must be provided to mechanically transfer torque between the mounting member and the secondary seal face while still allowing the necessary relative axial motion between the two. Any such torque transfer means almost inevitably involves some degree of circumferential lost motion. The flexible sealing member is generally not a good candidate for the torque transfer function, because it is flexible, and would wind up and be stressed. The addition of the flexible sealing member also creates two more interfaces that must be kept fluid tight, one with the secondary sealing face, and one with the mounting member. The first of these interfaces is generally no problem, as the secondary seal face can be fairly easily embedded in a thick portion of the elastomer. The interface between the mounting member and the elastomer is more difficult, and bears the brunt of the twisting stress inherent in the torque transfer. This has prompted some seal designs to actually provide an extra, sliding seal at that interface to absorb the lost motion. Another potential problem caused by interposing a flexible sealing member between the mounting member and the secondary seal face is that it is more difficult to maintain radial concentricity between the two, which affects seal wear and efficiency. Despite the plethora of competing designs available both commercially and on paper, those skilled in the art will generally agree that state of the art water pump seal integrity and life leave ample room for improvement.